The present invention relates generally to gas phase removal of oxides of nitrogen (NO.sub.x), and more particularly to a process for reduction of NO.sub.x in engine and power plant exhaust gases.
The invention is directed to a NO.sub.x control process applicable to a variety of industrial combustion sources, such as diesel power generators, gas turbines, glass and metal melting furnaces, incinerators, chemical processing plants, and refineries. Many of these sources have not been amenable to conventional emission control techniques. The process of the invention is particularly applicable to NO.sub.x control from lean burn engines, including diesel and spark ignited engines.
Lean burn engines pose particularly challenging problems in terms of NO.sub.x control. Diesel engines, for example, are the most efficient engines and thus produce the least amount of the greenhouse gas carbon dioxide (CO.sub.2) for energy delivered. However, the diesel engine also produces the most NO.sub.x for energy delivered and, consequently, its use as a power source is being discouraged.
Currently, large stationary diesel engines are used in a variety of industrial applications including cogeneration, natural gas pumping, and emergency power generation for nuclear power plants. These large engines are also fairly high emitters of NO.sub.x, NO.sub.x levels being typically 500-1000 parts-per-million by volume (ppm) in a flue gas with an oxygen (O.sub.2) concentration of 10%. For comparison, coal-fired utility boilers will emit NO.sub.x at levels of 250-600 ppm at an O.sub.2 level of 3%. Controlling NO.sub.x emissions from these diesel engines cannot be achieved by modifying aspects of engine operation, such as injection timing or the amount of pilot fuel, while maintaining satisfactory performance. Accordingly, in order to reduce polluting emissions, engine manufacturers set engine operating parameters to simultaneously lower NO.sub.x, and other undesirable exhaust constituents, such as carbon monoxide (CO), hydrocarbons and soot, thereby compromising engine performance. Moreover, enthusiasm for the adiabatic diesel engine, one that would use high temperature ceramic parts, has been mitigated by the knowledge that the high thermal efficiency associated with this engine will lead to increased NO.sub.x production.
In recent years, cogeneration has been implemented nationwide as a step towards energy conservation. Cogeneration projects have utilized a variety of prime movers and combustion sources, including diesel engines and gas turbines. Like diesel engines, exhaust from gas turbine engines has posed a challenge for emissions control. In addition, in some cases, these facilities burn waste fuels for which little emission control-information is available. Accordingly, while such relatively small cogeneration projects (in comparison to utility scale combustion systems) have contributed to energy conservation, they have, at the same time, posed emission control problems.
In addition to the engine and power plant emission problems, there has been increasing pressure to control emissions from industrial processes such as melting operations, refineries, chemical processing plants, and toxic and hazardous waste incinerators. Glass furnaces, for example, pose a particularly difficult problem for application of flue gas NO.sub.x control due to the potentially corrosive and noxious sodium sulfates present in their exhausts.
The above-noted combustion sources pose particular problems for emissions control due to the temperature, and gaseous and particulate content of their exhausts. Thus, there is a need to address the air pollution control problems facing such combustion sources.
Other than expensive wet scrubbing systems, post-combustion technologies have focused on non-selective gas phase NO.sub.x reduction, ammonia based selective catalytic reduction (SCR), and selective non-catalytic reduction (SNCR) using ammonia (NH.sub.3), urea, cyanuric acid, iso-cyanate, hydrazine, ammonium sulfate, atomic nitrogen, malamine, methyl amines or bi-urates.
The first of these technologies, non-selective gas phase NO.sub.x reduction, involves injection of a NO.sub.x reducing agent, or reductant, such as methane or another hydrocarbon. However, the compound will not only react with NO.sub.x but also with oxygen present in the exhaust. This requires that sufficient reductant be injected to consume both the NO.sub.x and the oxygen. This treatment approach is most applicable to combustion systems operating at fuel/air ratios near stoichiometric, where low less fuel is required to reduce the oxygen. This approach is inappropriate for diesel engines, for example, where oxygen concentrations vary from approximately 8-14%. Moreover, the considerable addition of fuel required for this method renders it prohibitively expensive in many applications.
A second approach, selective catalytic NO.sub.x reduction (SCR), involves the reduction of NO.sub.x by ammonia (NH.sub.3) over a catalyst. Catalysts are typically titanium dioxide and vanadium pentaoxide, or zeolites. The catalysts operate in a temperature region between about 550.degree. F.-800.degree. F. While selective catalytic NO.sub.x reduction is capable of high levels of NO.sub.x removal, the process is not without drawbacks: Catalyst cost is high, catalysts oxidize sulfur in the fuel to sulfate (SO.sub.3), and reactions between SO.sub.3 and residual ammonia (NH.sub.3) can cause problems with downstream equipment. Furthermore, the temperature of the exhaust must be in the range of 550.degree.-800.degree. F., and since the catalyst is subject to poisoning and deactivation, catalyst life is uncertain. In addition, catalyst induced pressure drop can interfere with engine operation. Finally, the spent catalyst may be classified as a hazardous substance subject to strict transportation and disposal regulations.
A number of the issues enumerated above are particularly applicable to the diesel or gas turbine engines and other industrial combustion processes. Typical engine exhaust temperatures for these combustors are on the order of 1000.degree. F. To apply SCR, heat extraction is required to provide the proper temperature for reaction, in addition to heat recovery downstream of the SCR reactor to provide overall energy recovery. Thus, in a combined cycle system the SCR reactor would be in the middle of the waste heat recovery boiler.
More importantly, during some operating modes, diesel engines, gas turbines and other industrial combustors can emit carbon particulates or lubrication oil, which, if carried over in the exhaust, can lead to catalyst deactivation. This can make the process both economically unattractive and operationally difficult. Thus, while SCR is being applied to boiler exhausts and gas turbine exhausts, its application to the exhausts of such combustors on a widespread basis has met with resistance.
A third approach is selective non-catalytic reduction of NO.sub.x (SNCR). A number of processes fall within this category, each involving the injection of a chemical that selectively reacts, in the gas phase, with NO.sub.x in the presence of oxygen at a temperature greater than 1150.degree. F. Chemical NO.sub.x reduction agents used in such processes include ammonia (NH.sub.3), urea (NH.sub.2 CONH.sub.2), cyanuric acid (HNCO).sub.3, iso-cyanate, hydrazene, ammonium sulfate, atomic nitrogen, malamine, methyl amines, or bi-urates.
While the detailed chemistry of the processes listed above varies, the overall chemistry is best illustrated by considering the reactions between NH.sub.3 and the oxide of nitrogen, nitric oxide (NO). Upon injection, the NH.sub.3 breaks down to form NH.sub.2 radicals primarily by the reaction EQU NH.sub.3 +OH.fwdarw.NH.sub.2 +H.sub.2 O (1)
The NO is then primarily removed by reaction with NH.sub.2 radicals according to the following: EQU NH.sub.2 +NO.fwdarw.N.sub.2 +H.sub.2 O (2) EQU NH.sub.2 +NO.fwdarw.N.sub.2 +H+OH (3)
Another oxide of nitrogen present in combustion exhaust is nitrogen dioxide (NO.sub.2). Removal of NO.sub.2 also occurs according to the same chemistry, since NO.sub.2 is converted to NO by the following reaction: EQU NO.sub.2 +H--NO+OH (4)
The OH is subsequently replenished through the reverse of reaction (1) and the following H.sub.2 -O.sub.2 system reactions: EQU H+O.sub.2 .fwdarw.OH+O (5) EQU O+H.sub.2 .fwdarw.OH+H (6) EQU O+H.sub.2 O.fwdarw.OH+OH (7)
Since reactions (5) and (7) are strongly temperature dependent, OH is not replenished fast enough to convert NH.sub.3 to NH.sub.2 at temperatures below approximately 1500.degree. F., in the available reaction time. Nitric oxide (NO) removal efficiency thus falls off in direct proportion to the OH concentration.
At high temperatures, nitric oxide conversion also decreases due to the increasing efficiency of NO formation reactions relative to NO removal reactions. Since increased OH is present at these temperatures, an increasing fraction of the available NH.sub.2 reacts with 0H to produce NH, as opposed to reacting with the NO to produce molecular nitrogen. The resulting NH radicals subsequently follow several different reaction paths which are summarized below: EQU NH+NO.fwdarw.N.sub.2 +OH (8) EQU NH+NO.fwdarw.N.sub.2 O+H (9) EQU NH+OH.fwdarw.HNO+H (10) EQU NH+CO.sub.2 .fwdarw.HCO+CO (11) EQU HCO+NH.sub.2 .fwdarw.NO+NH.sub.3 ( 12) EQU HNO+M.fwdarw.NO+H+M (13)
As several of these paths form NO, the process efficiency is reduced. The net result is that the NO reduction occurs over a moderately narrow temperature range from about 1160.degree. F. to about 1880.degree. F. (900-1300 K.; 620.degree.-1020.degree. C.), and at about 960.degree. C. (1750.degree. F.) under normal diesel operating conditions, as illustrated in FIG. 1. FIG. 1 is a plot of NO removal by the noted reductants versus temperature. The decrease in performance at low temperatures results from decreased formation of NH.sub.2 radicals, while at high temperatures, the NH.sub.2 radicals can lead to NO formation.
FIG. 1 also shows that other reductants, such as urea (NH.sub.2 CONH.sub.2) and cyanuric acid ((HNCO).sub.3), work in a similar fashion to NH.sub.3, although their detailed chemistry is somewhat different. However, these reductants can also produce iso-cyanate (HNCO) which will react to form the cyano radical NCO. The NCO then reacts with NO to form byproducts, such as nitrous oxide (N.sub.2 O). This formation of N.sub.2 O is a serious negative byproduct of the chemical processes that use these reductants.
Another consequence of the process is the emission ("slip") of some NH.sub.3. FIG. 2 shows the relationship between NO removal with urea and NH.sub.3 slip. Curve A represents NH.sub.3 content in exhaust. Curve B represents the effectiveness of NO.sub.x removal as a function of % NO.sub.x removed. Both components are plotted versus exhaust temperature. As the temperature increases, NH.sub.3 slip is reduced. However, NO.sub.x removal peaks at a temperature of about 960.degree. C. (1750.degree. F.) in this example, at which temperature there is still significant NH.sub.3 slip. The present invention also provides a means of reducing NH.sub.3 slip by operating above the peak temperature for NO.sub.x removal downstream from the initial NO.sub.x reduction zone.
The temperature at which the NO.sub.x removal reactions occur can be lowered by the presence of other species; specifically, species producing OH radicals. Hydrogen (H.sub.2) may be used to alter optimum temperatures. As the amount of hydrogen is increased, the temperature for maximum NO.sub.x removal drops from 960.degree. C. (1760.degree. F.) to 700.degree. C. (1290.degree. F.). This occurs since H.sub.2 produces OH radicals via reaction (6).
In an exhaust with modest amounts of CO and hydrocarbons present, the carbon monoxide and hydrocarbons can initiate a chain branching sequence which consists of the following set of reactions: EQU CO+OH.fwdarw.CO.sub.2 +H (14) EQU H+O.sub.2 .fwdarw.OH+O (15) EQU O+H.sub.2 .fwdarw.OH+H (16)
Thus, in such an exhaust, the peak temperature for selective gas phase NO.sub.x removal occurs at temperatures less than 960.degree. C. The O.sub.2 concentration also has a secondary effect on these processes.
One of the difficulties in applying SNCR processes is the availability of appropriate temperatures. For engines such as diesels or gas turbines, exhaust temperatures of about 1000 F are too low to merely inject NH.sub.3 or another chemical. Finding the appropriate temperature within the engine cycle is either not feasible, or sufficiently complicates the process to render it inappropriate. In other industrial processes, such as metal or glass melting, refining or incineration, similar situations occur.
One option is to reheat the exhaust gases to a temperature appropriate for the selective gas phase NO.sub.x reduction reactions to occur. However, without heat recovery, this results in a significant energy penalty that will in many cases be intolerable.
Another problem encountered with conventional SNCR is incomplete mixing of the chemical reductant with the exhaust gases during the reduction reaction with the result that significant amounts of NO.sub.x are not removed from the exhaust.
Consequently, a NO.sub.x removal scheme that is energy efficient, cost effective, eliminates catalyst replacement, improves reductant distribution, and directly decomposes NO.sub.x to N.sub.2 and H.sub.2 O is urgently needed.
Accordingly, an object of the present invention is to provide an SNCR process for NO.sub.x that provides complete mixing of NO.sub.x reductant with exhaust gases prior to NO.sub.x reduction.
It is a further object of the present invention to provide an SNCR process for NO.sub.x with reduced energy consumption.
It is yet another object of the present invention to provide an SNCR process for NO.sub.x that eliminates the N.sub.2 O and other byproduct emissions and NH.sub.3 slip associated with SNCR.
It is still another object of the present invention to provide an SNCR process for NO.sub.x with no external fuel or heating requirements.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the claims.